The present invention relates to optical fiber communication systems, and more particularly to a signal source for use in transmitting a modulated information signal over a fiber optic communication link having high fiber dispersion.
Various communication systems, such as cable television (CATV) systems, currently distribute information signals via coaxial cable. The replacement of coaxial cable with optical fiber transmission lines in such communication systems has become a high priority. Production single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems.
In a CATV system, the information that is communicated is contained in a television signal. The use of a television signal comprising amplitude modulated vestigial sideband video subcarriers (VSB-AM) is preferred in the distribution of cable television signals due to the compatibility of that format with the standards of the National Television Systems Committee (NTSC) and the ability to provide an increased number of channels within a given bandwidth. An undesirable characteristic of VSB-AM transmission, however, is that it requires a much higher carrier-to-noise ratio (CNR) than other techniques, such as frequency modulation or digital transmission of video signals. Generally, a CNR of at least 40 dB is necessary to provide clear reception of VSB-AM television signals. Although VSB-AM is currently preferred for the reasons stated above, it is expected that as advances are made in technology, the transmission of FM modulated video over fiber optic communication systems will also become economically feasible and desirable.
The advent of commercially available rare earth fiber amplifiers, such as an Erbium fiber amplifier, has led to the consideration of systems that transmit VSB-AM television signals or FM modulated video signals at the optical wavelength of 1.5 .mu.m (microns), which is the wavelength at which Erbium fiber amplifiers operate. However, most fiber links currently installed utilize fiber that has minimum dispersion near 1310 nm (about 1.3 .mu.m) instead of the 1.5 .mu.m operating wavelength of an Erbium fiber amplifier. In analog communication systems, this dispersion at 1.3 .mu.m leads to high levels of distortion, and in particular to very high levels of second order distortion when laser sources are used that exhibit optical chirp. Optical chirp is defined as a rapid change in the emission wavelength of an optical source upon modulation of the source. For example, distributed feedback (DFB) and Fabry-Perot laser diodes exhibit simultaneous AM and FM modulation in the optical domain. Typical values for the FM modulation of DFB lasers is on the order of 200 MHz/mA. Actual numbers may range from 20 MHz to 800 MHz/mA, depending on the laser design. The general form for such a laser can be expressed as: EQU E[m(t)]=E.sub.o [m(t)]cos[w.sub.c t+.beta.sinW.sub.m (t)]
where
m(t) represents AM modulation, PA1 W.sub.c represents the optical carrier, PA1 W.sub.m represents the modulation frequency, and PA1 .beta. represents the FM modulation index.
A laser exhibiting such characteristics is said to be both intensity modulated (AM) and frequency modulated (FM). Because the cost of fiber which has minimum dispersion at 1.5 .mu.m is higher and since some fiber having minimum dispersion near 1.3 .mu.m is already in use in communication networks, such as CATV, it would be desirable to utilize fiber having minimum dispersion near 1.3 .mu.m. However, applicant has discovered that if a laser having chirp (i.e., frequency modulation as well as amplitude modulation) is utilized in such a system, high levels of second order distortion will result.
If an optical amplifier is added to a optical communication system to obtain additional optical link budget, high levels of distortion, primary second order, will result if the gain spectrum of the optical amplifier is not flat. Such distortion is unacceptable in a practical system.
It is possible to provide electronic compensation to deal with the distortion generated by fiber dispersion and/or an optical amplifier having a nonzero gain slope in such a system. However, the amount of distortion compensation required is a function of fiber dispersion at the source laser operating wavelength, the length of the link fiber, and the detected signal power.
Accordingly, it would be advantageous to provide an alternative to electrical compensation in a fiber optic signal source having an optical amplifier with a nonzero gain slope. Any such alternative must be reliable and economically viable, and would advantageously also accommodate systems having link fibers with high fiber dispersion.
The present invention provides a signal source incorporating an optical amplifier without a flat (i.e., zero slope) gain spectrum, for transmitting modulated information signals over fiber optic communication links, which enjoys the aforementioned advantages.